REVIEW OF THE PRIOR ART
In U.S. Pat. No. 3,902,581, issued Sept. 2, 1975 to Hans G. Knudsen, there is disclosed an electromagnetic brake or clutch in which an electromagnet having an annular core is flanked by two armature members which are drawn towards the core on energization of the electromagnet so as to clamp two stacks of plates of a multiplate clutch or brake between the armatures and the core and thus cause engagement of the brake or clutch. In U.S. Pat. No. 2,698,679, issued Jan. 4, 1955 to J. M. Vernhes, two armatures in the form of relatively axially movable discs are mounted for rotation with a shaft and may be drawn into braking engagement with the core of an electromagnet when the latter is energized. A clutch and a multi-disc brake are also described. The electromagnet core comprises an internal or external ferromagnetic disc or spider through which clutching or braking torque is transmitted, an outer and inner ferromagnetic annulus between which and the disc or spider a magnet coil is housed, and a non-magnetic metallic member uniting the annulus to the disc or spider to produce a structurally stong and rigid unit, which is however relatively complex to produce and must be exchanged as a whole in the event of a coil defect.